1. Field of the Invention
The present invention relates to a method of producing an optical film used for an optical antireflective film and using a capsule having a hollow structure and a coating liquid for a production of an optical film, in particular, an optical film having a particularly excellent strength and a low refractive index.
2. Description of the Related Art
Antireflection methods with an antireflective film include a method involving forming a thin film having a thickness close to a light wavelength to reduce a reflectance by an interference effect of light, and a treatment method involving forming fine irregularities using fine particles and scattering a reflective image by light scattering to make an outline vague.
The method of reducing a reflectance by an interference effect of light includes a method involving forming a transparent material having a low refractive index, such as magnesium fluoride, calcium fluoride, or silica, into a film with a thickness of several tens of nanometers to several hundred nanometers on a substrate. Meanwhile, halogenated compounds of elements of Group 2 are known to have a low refractive index.
For example, the halogenated compounds of elements of Group 2 include magnesium fluoride, which has a refractive index of 1.38. However, by forming magnesium fluoride into a thin film with a thickness of around a hundred nanometers on a surface of a lens having a refractive index of about 1.50, the reflectance of the lens can be reduced to 2% or less in a visible region of from 400 to 700 nanometers.
In order to make a reflectance of the lens lower, a refractive index must be made lower. For accomplishing the purpose, a refractive index can be lowered by forming voids in a film of the above-mentioned magnesium fluoride, calcium fluoride, or silica, because the refractive index of air is 1.0. For example, when a thin film of magnesium fluoride having a refractive index of 1.38 includes voids at 30% (volume), the refractive index of the film can be lowered to 1.27. Further, in general, as a refractive index becomes lower, a reflectance can be reduced more.
As a method of incorporating voids efficiently, there is a report that fine particles of magnesium fluoride are produced, the fine particles with a binder are formed into a film, and voids are formed in gaps between the particles, thereby being capable of reducing a refractive index (see International Publication No. 02/018982).
However, in International Publication No. 02/018982, in order to increase the voids for lowering the refractive index, the diameter of the fine particles of magnesium fluoride must be made larger, and an amount of the binder must be made smaller. As a result, reduction in the amount of the binder contributes to weaker binding forces between particles, between a particle and a substrate, and between a particle and its lower film, resulting in detachment of a particle from a substrate.